During knee resurfacing arthroplasty, commonly called knee replacement surgery, the distal surfaces of the femur are cut away and replaced with an metal cap to simulate the baring surfaces of the femur. The proximal surface of the tibia is modified in a similar way, to provide a metal-backed plastic bearing surface. The metal femoral component of the new "artificial joint" transfers the patient's weight to the tibial component such that the joint can support the patient's weight and provide a near-normal motion of the knee joint.
Several studies have indicated that the long term survival of such an artificial knee joint is dependant on how accurately the components of the knee joint are implanted with respect to the weight bearing axis of the patient's leg. In a correctly functioning knee, the weight bearing axis passes through the centre of the head of the femur, the centre of the knee and the centre of the ankle joint. This weight bearing axis is typically located by analyzing an X-ray image of the patient's leg, taken while the patient is standing.
The X-ray image is used to locate the centre of the head of the femur and to calculate its position relative to selected landmarks on the femur. The selected landmarks were then found on the patient's femur during surgery and the calculations used to estimate the actual position of the femoral head. These two pieces of information are used to determine the correct alignment of the weight bearing axis for the femur. To completely define the correct position for the femoral component of the knee prosthesis, the correct distance between the centre of the femoral head and the knee joint and the rotation of the knee joint about the weight bearing axis must be established. These two pieces of information are determined from landmarks on the distal portion of the femur. The correct alignment for the tibial component of the knee prosthesis is determined by finding the centre of the ankle joint and relating its position to landmarks on the tibia. This point and the centre of the proximal tibial plateau are used to define the weight bearing axis of the tibia. The correct distance between the ankle joint and the knee joint and the rotation of the knee joint about the weight bearing axle are determined by reference to the distal portion of the femur and landmarks on the tibial plateau.
Various mechanical alignment instruments am used to assist the surgeon in making cuts on the distal femur and proximal tibia which will allow the femoral and tibial components of the new knee joint to be attached to the femur and tibia. These mechanical alignment instruments permit the surgeon to fix cutting guides in place with respect to the selected landmarks on the bones so that the cuts will be correctly oriented with respect to the weight bearing axis determined from the X-ray image.
There are two general types of alignment instruments in common use. These are intramedullary and extramedullary alignment systems. Intramedullary alignment systems use the inside of the femur or tibia, the medullary canal, as one of the selected landmarks for establishing alignment. Extramedullary alignment systems use only the external surfaces of the body to establish alignment.
A typical extramedullary alignment system requires the surgeon to visually align a slender rod with the centre of the knee and the centre of the femoral head for alignment of the femoral component, then align a similar rod with the centre of the ankle and the centre of the tibial plateau for alignment of the tibial component. The centers of the femoral head and ankle are found by either palpitation or established with an intraoperative X-ray. If correctly placed, the rods will lie parallel to, and offset from the weight bearing axis. Once aligned, the rods are used as a guide to fix the location of the cutting guides with respect to the femur and tibia so that the cuts can be performed.
A typical intramedullary alignment system requires the surgeon to insert a rod into the medullary canal of the femur and tibia. If properly placed, these rods should lie on the axis of the bones. In the case of the tibia, the weight bearing axis is very close to the axis of the bone. In the case of the femur the axis of the bone is quite different from the weight bearing axis due to the offset nature of the hip joint, and this difference must be measured from the pre-operative X-ray and used to correct the alignment of the femoral cutting jigs.
Both intramedullary and extramedullary approaches to alignment have numerous inherent drawbacks and sources of error. Extramedullary alignment depends on accurate visual estimation of the alignment of the extramedullary rods. Location of the femoral head by palpitation is difficult and error prone, particularly with obese patients. Use of intraoperative X-rays improves the result somewhat, but this is time consuming and exposes the patient and operating room personnel to radiation. X-rays are also subject to distortion and require visual interpretation and estimation to correctly analyze, as they offer only one planar view in two dimensions.
Intramedullary alignment approaches provide only slightly better results, in that the knee joint alignment is still determined by estimating the difference between the bone axis and the weight bearing axis from a potentially distorted X-ray image. In addition, intramedullary rods must be introduced very carefully, not only to make sure they align correctly with the medullary canal, but also to make sure that the insertion of the rods does not create an embolism, which could kill or seriously injure the patient.
An ideal alignment system finds the weight bearing axis of the patient's leg directly, without the need for preoperative or intraoperative X-rays, estimation, calculation, location of hidden or obscured landmarks, or surgical intervention outside of that required for access to the knee joint surfaces. The ideal alignment system depends only on the accepted definition that the weight bearing axis passes through the centre of the head of the femur, the centre of the knee joint and the centre of the ankle, in order to locate the weight bearing axis.